In the past, sensor systems have been commonly employed in cranes to detect overloads affecting the boom and cable of the crane. However, such load sensors have had limited application in the past in detecting and preventing excessive torque applied to the base or turret of such a crane.
When cranes lift large amounts of weight, a great amount of torque is applied to the base of the crane. The amount of torque applied will be a function of the weight being lifted, and the distance from the base of the crane to the cable attached to the load, measured perpendicularly to the cable. In many instances, a destructive amount of torque can be applied to the base of the crane without causing excessive strain and stresses on the lifting boom. This is particularly true for the smaller truck-mounted cranes. Certain U.S. patents have described overload sensors and limit switches that are associated with the base of cranes. U.S. Pat. No. 3,357,571, issued to J. H. Boughton, described a microswitch that is responsive to the angular movement of the superstructure of a tractor-mounted crane. A microswitch is attached to two horizontal flanges. These horizontal flanges correspond to the superstructure and base of the crane. When excessive loads are being lifted by the crane, the angular separation of these horizontal flanges will actuate the microswitch so as to sound a warning.
U.S. Pat. No. 1,614,575, issued to C. T. Siebs, shows a limit switch arranged about the base of the cab of a tractor-mounted crane. This limit switch is responsive to rotational movements of the cab relative to the platform. This limit switch, however, is used only for sensing the angular position of the crane relative to the tracks. It did not have application for sensing base-affecting bending loads caused by excessive torque. Several other patents have issued that relate to sensors that detect overloads affecting crane systems. These patents include U.S. Pat. No. 2,910,189; U.S. Pat. No. 3,824,578; U.S. Pat. No. 4,067,446; and U.S. Pat. No. 4,427,121.
It is an object of the present invention to provide an improved crane that has a sensor for detecting excessive torques about the base of the crane.
It is another object of the present invention to provide a relatively inexpensive and easy-to-manufacture overload sensor system for truck-mounted cranes.
It is a further object of the present invention to provide an overload sensor for a crane that both detects and prevents destructive overloads during the lifting or moving of objects.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.